In pictures of the interiors of North American churches, even Catholic churches, one often sees the national flag in the Sanctuary to one side of the Altar.
I have never* seen this in an English church of any denomination. The most you might find would be the ancient 'laid up' colours of dear, long-forgotten regiments hanging from a dark and dusky ceiling, deliberately left to fall apart in cobwebs and sanctity.
I wonder when the American custom arose. To my quaint European instincts it seems an incomprehensible intrusion of transient terrestrial territorial politics into the Place of Eternity.
During the last war ... imagine German POWs being marched to Mass in an English church ... how easily could they have worshipped if the British Flag had been hanging aggressively in front of their eyes? And vice versa.
I believe the pro-Hitler 'German Christians' did it in the 1930s.
Do Ulster Protestants do it?
Do Canadians do it?
Do North American Orthodox or Eastern Catholics do it?
*Exception: In S James, Spanish Place, once the Spanish Embassy Chapel, there is, in one of the side aisles, the Spanish Royal Standard, handily placed so that, if the pp gets a message that His Most Catholic Majesty is about to pay a surprise visit, he can haul it up a rapid flagpole ...